Generally, liquid crystal displays (LCDs) do not work well with direct current (DC) voltages. A graph of transmission versus voltage of an LCD is shown in FIG. 1, showing high transmission with zero voltage and low transmission with either positive or negative voltage. To drive the LCD to black, a positive voltage cannot be placed on the LCD. A steady state DC voltage may damage the display by, for example, causing contaminants to plate one side or the other of the liquid crystal cell. To preserve zero (0) DC (DC restore) and prevent damage, generally the voltage applied to the LCD is flipped back and forth (alternated) between high-black, low-black, high-black, low-black.
There are different scenarios for preserving zero (0) DC, as shown in the series of succeeding frames of FIGS. 2A–2D. One scenario uses column inversion as shown in FIG. 2A, where one frame is written with all the columns having alternating polarity, positive-negative, positive-negative. In the next frame all the columns are written negative-positive, negative-positive. In the succeeding frame, all the columns are again written positive-negative, positive-negative. As shown in FIG. 2B, frame inversion can be used where the first frame is written with all positives and the next frame is written with all negatives. The succeeding frame is again written with all positives. As shown in FIG. 2C, pixel inversion can be used which produces a checkerboard like effect in the first frame and an inverted effect in the second frame. In the third frame, the checkerboard like effect matches that of the first frame. Lastly, as shown in FIG. 2D, row inversion can be used where all the rows are alternating polarity, positive-negative, positive-negative. In the next frame all the rows are written negative-positive, negative-positive. In the third frame, the rows are again written positive-negative, negative-negative.